U.S. patent number 7,871,198 [Application Number 11/678,901] was granted by the patent office on 2011-01-18 for high-temperature thermocouples and related methods.
This patent grant is currently assigned to Battelle Energy Alliance, LLC. Invention is credited to Keith G. Condie, Darrell L. Knudson, Joy L. Rempe, S. Curt Wilkins.
United States Patent |
7,871,198 |
Rempe , et al. |
January 18, 2011 |
**Please see images for:
( Certificate of Correction ) ** |
High-temperature thermocouples and related methods
Abstract
A high-temperature thermocouple and methods for fabricating a
thermocouple capable of long-term operation in high-temperature,
hostile environments without significant signal degradation or
shortened thermocouple lifetime due to heat induced
brittleness.
Inventors: |
Rempe; Joy L. (Idaho Falls,
ID), Knudson; Darrell L. (Firth, ID), Condie; Keith
G. (Idaho Falls, ID), Wilkins; S. Curt (Idaho Falls,
ID) |
Assignee: |
Battelle Energy Alliance, LLC
(Idaho Falls, ID)
|
Family
ID: |
39715862 |
Appl.
No.: |
11/678,901 |
Filed: |
February 26, 2007 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20080205483 A1 |
Aug 28, 2008 |
|
Current U.S.
Class: |
374/179; 136/232;
136/230; 374/208 |
Current CPC
Class: |
H01L
35/34 (20130101); G01K 7/02 (20130101); Y10T
29/49085 (20150115) |
Current International
Class: |
G01K
7/00 (20060101); G01K 1/00 (20060101) |
Field of
Search: |
;374/179,208
;136/232,230 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
International Search Report and Written Opinion of the
International Searching Authority, International Application No.
PCT/US07/83130, International Filing Date Oct. 31, 2007. cited by
other .
Rempe, J.L., et al., Dec. 2006. "Thermocouples For High-Temperature
In-Pile Testing," Nuclear Technology, vol. 156, Dec. 2006, pp.
320-331. cited by other .
Rempe, J.L., and Wilkins, S.C., "Specialized Thermocouples for High
Temperature In-situ Applications," Jan. 2005, Idaho National
Engineering and Environmental Laboratory. cited by other .
Rempe, J.L. et al., "Thermocouples for High Temperature In-Pile
Testing," Idaho National Laboratory. cited by other .
Rempe, J.L., "High Temperature Thermocouples For In-Pile
Applications," The 11.sup.th International Topical Meeting on
Nuclear Reactor Thermal-Hydraulics (NURETH-11), Avignon, France,
Oct. 2-6, 2005. pp. 1-12. cited by other .
Rempe, J. et al., "Development and Evaluation of High Temperature
Thermocouples for In-Pile Applications," American Nuclear Society
Winter Meeting, Washington, D.C., Nov. 13-17, 2005. cited by other
.
Rempe, J. et al., "Development and Evaluation of High Temperature
Thermocouples for In-Pile Applications," American Society of
Testing and Measurement Committee E20 Meeting, Dallas, TX, Nov.
2005. cited by other .
Fanciullo, Salvatore, "Thermocouple Development," AEC Research and
Development Report (entire report), Issued Mar. 5, 1964. cited by
other .
Holmes, Robert R., "Development of High Temperature Sensors," AEC
Research and Development Report (entire report), Issued Jun. 30,
1961. cited by other .
Fornwalt, D.E., et al., "A Study of the Compatibility of Selected
Refractory Metals with Various Ceramic Insulation Materials,"
presented at Electron Microprobe Symposium of Electromechanical
Society Meeting, Washington, D.C., Oct. 1964. cited by
other.
|
Primary Examiner: Caputo; Lisa M
Assistant Examiner: Jagan; Mirellys
Attorney, Agent or Firm: TraskBritt
Government Interests
GOVERNMENT RIGHTS
This invention was made with government support under Contract No.
DE-AC07-05ID14517 awarded by the United States Department of
Energy. The government has certain rights in the invention.
Claims
The invention claimed is:
1. A method of manufacturing a thermocouple, comprising: forming a
thermocouple junction between a first thermoelement wire and a
second thermoelement wire formed from thermoelectrically dissimilar
materials by swaging a metallic tube onto a first end of the first
thermoelement wire and a first end of the second thermoelement
wire; threading the first thermoelement wire through a first
longitudinal passageway formed in a first electrical insulating
material and threading the second thermoelement wire through a
second longitudinal passageway formed in the first electrical
insulating material; disposing the first electrical insulating
material, a second electrical insulating material, and a third
electrical insulating material having a different density than the
first electrical insulating material and the second electrical
insulating material in a sheath, comprising: interfacing the third
electrical insulating material with the second electrical
insulating material; and positioning the thermocouple junction
within a void formed in the first electrical insulating material
and another void formed in the second electrical insulating
material; swaging the sheath so that the first electrical
insulating material and the second electrical insulating material
contact the first thermoelement wire, the second thermoelement
wire, and the thermocouple junction; determining a location of the
interface between the second electrical insulating material and the
third electrical insulating material; cutting the thermocouple
between the thermocouple junction and the interface between the
second electrical insulating material and the third electrical
insulating material to form a sheath end; and capping the sheath
end with an end plug.
2. The method of claim 1, further comprising heating the first
electrical insulating material, the second electrical insulating
material, and the third electrical insulating material at a
sufficient temperature and time to remove moisture contained
therein.
3. The method of claim 1, further comprising forming the first
electrical insulating material and the second electrical insulating
material from hafnia.
4. The method of claim 1, further comprising forming the third
electrical insulating material from alumina.
5. The method of claim 1, wherein determining a location of the
interface between the second electrical insulating material and the
third electrical insulating material comprises determining the
location of the interface between the second electrical insulating
material and the third electrical insulating material with an x-ray
imaging system.
6. The method of claim 1, wherein capping the sheath end with an
end plug comprises laser welding a niobium plug onto the sheath
end.
7. The method of claim 1, further comprising heating the first
thermoelement wire, the second thermoelement wire, the first
electrical insulating material, the second electrical insulating
material, and the third electrical insulating material at a
sufficient temperature and time to remove moisture contained
therein.
8. The method of claim 7, further comprising heating the
thermocouple to at least 100.degree. C. above a planned service
temperature of the thermocouple for at least three hours to
stabilize a grain structure of the thermocouple.
9. The method of claim 1, further comprising forming the first
thermoelement wire from molybdenum doped with elements selected
from the group consisting of tungsten, potassium, and silicon.
10. The method of claim 9, wherein forming the first thermoelement
wire from molybdenum doped with elements selected from the group
consisting of tungsten, potassium, and silicon comprises forming
the molybdenum with doped elements that are present in amounts
ranging from between 100 parts per million to 300 parts per
million.
11. The method of claim 1, further comprising forming the second
thermoelement wire from a niobium and zirconium alloy.
12. The method of claim 11, wherein forming the second
thermoelement wire from a niobium and zirconium alloy comprises
forming the niobium and zirconium alloy from 99% by weight niobium
and 1% by weight zirconium.
13. The method of claim 1, further comprising forming the sheath
from a niobium and zirconium alloy.
14. The method of claim 13, wherein forming the sheath from a
niobium and zirconium alloy comprises forming the niobium and
zirconium alloy from 99% by weight niobium and 1% by weight
zirconium.
15. A method of fabricating a thermocouple, comprising: forming a
thermocouple junction by swaging an end of a first thermoelement
wire and an end of a second thermoelement wire formed from
thermoelectrically dissimilar materials with a metallic tube;
heating a first insulating material, a second insulating material,
and a third insulating material at a sufficient temperature and
time to remove moisture and impurities contained therein; threading
the first thermoelement wire through a first passageway formed in
the first insulating material and threading the second
thermoelement wire through a second passageway formed in the first
insulating material; disposing the first insulating material, the
second insulating material, and the third insulating material
within a metallic sheath, comprising: forming an interface between
the second insulating material and the third insulating material;
and positioning the thermocouple junction within a void formed in
the first insulating material and another void formed in the second
insulating material; swaging the sheath so that the first
insulating material and the second insulating material contact the
first thermoelement wire, the second thermoelement wire, and the
thermocouple junction; cutting the thermocouple at a location
between the thermocouple junction and the interface between the
second insulating material and the third insulating material to
form a sheath end; and capping the sheath end with an end plug.
16. The method of claim 15, further comprising heating the capped
thermocouple at a sufficient temperature and time to stabilize a
grain structure of the thermocouple.
17. The method of claim 15, further comprising forming the first
insulating material and the second insulating material from
hafnia.
18. The method of claim 15, further comprising forming the third
insulating material from alumina.
19. The method of claim 15, further comprising determining the
location of the thermocouple junction and the interface between the
second insulating material and the third insulating material by
x-ray imaging.
20. The method of claim 15, wherein capping of the sheath end with
an end plug comprises laser welding a niobium plug onto the sheath
end.
21. The method of claim 15, further comprising forming the sheath
from a niobium and zirconium alloy.
22. A thermocouple produced by the method of claim 15.
23. The method of claim 15, further comprising forming the first
thermoelement wire from molybdenum doped with elements selected
from the group consisting of tungsten, potassium and silicon.
24. The method of claim 23, wherein forming the first thermoelement
wire from molybdenum doped with elements selected from the group
consisting of tungsten, potassium and silicon comprises forming the
molybdenum with doped elements that are present in amounts ranging
from between 100 to 300 parts per million.
25. The method of claim 15, further comprising forming the second
thermoelement from a niobium and zirconium alloy.
26. The method of claim 25, wherein forming the second
thermoelement from a niobium and zirconium alloy comprises forming
the niobium and zirconium alloy from 99% by weight niobium and 1%
by weight zirconium.
27. A high-temperature thermocouple, comprising: a thermocouple
junction between a first thermoelement wire comprising a doped
molybdenum and a second thermoelement wire comprising a niobium and
zirconium alloy; a first electrical insulating material comprising:
a first longitudinal passageway and a second longitudinal
passageway formed therein, wherein the first thermoelement wire is
at least partially disposed in the first longitudinal passageway
and the second thermoelement wire is at least partially disposed in
the second longitudinal passageway; and a void formed in the first
electrical insulating material for receiving a portion of the
thermocouple junction; a second electrical insulating material
comprising a void for receiving a portion of the thermocouple
junction; a sheath comprising a niobium and zirconium alloy having
the first electrical insulating material, material and the second
electrical insulating material at least partially disposed therein,
wherein the thermocouple junction is at least partially positioned
within the void of the first electrical insulating material and the
void of the second electrical insulating material, and wherein the
first electrical insulating material and the second electrical
insulating material is at least partially in contact with the first
thermoelement wire, the second thermoelement wire, and the
thermocouple junction; and an end plug disposed on an end of the
sheath proximate to the thermocouple junction.
Description
TECHNICAL FIELD
This invention relates to a thermocouple design and fabrication and
more specifically to a high-temperature thermocouple capable of
long-term operation in hostile high-temperature environments
without significant signal degradation.
BACKGROUND
The accurate measurement of temperatures between 1100.degree. C.
and 1700.degree. C. is important to the safe, efficient and
economical operation of many industries such as electrical power
production, processing and refining of chemicals, the fabrication
of steel and other metals, and production of glass and ceramic
materials. Accurate temperature measurement over time can also be
critical to the operation of industrial machinery such as jet
engines, nuclear reactors, gasification units, incinerators, and
gas turbines. In such temperature environments, thermocouples are
the most widely used industrial temperature sensors because they
are rugged, affordable and accurate--at least initially.
Unfortunately, after installation all commercial thermocouples are
unstable in this temperature range and prone to decalibration or
"drift," providing increasingly unreliable and unpredictable
readings as they age. As operating temperatures and thermal cycles
increase, the performance of these thermocouples decreases.
Together, these factors often result in costly redundant instrument
clusters, sensor failures, downtime and potential accidents due to
undetected overheating. For temperatures above 1100.degree. C. in
radiation environments, such as in high-temperature nuclear test
reactors, conventional thermocouples are incapable of stable and
accurate operation.
The thermocouple of the present invention overcomes the two most
critical thermocouple issues plaguing high-temperature operations;
signal drift and sensor longevity. The first problem with all
conventional thermocouple sensors is that they are subject to
decalibration. The uncertainties surrounding decalibration are
difficult to quantify, but elevated temperatures and longer
operating times inevitably result in increasingly unreliable
measurements. Standard thermocouples drift appreciably after a few
hundred hours of use, making accurate temperature measurement and
high-temperature process control difficult without frequent sensor
change out. For high-temperature nuclear applications there are
even greater limitations. Currently there are no high-temperature
thermocouples capable of withstanding neutron flux in nuclear
fission reactors or, potentially one day, fusion reactors. The key
to minimizing drift lies in selecting thermocouple materials with
properties that do not interact with each other or appreciably
change during use.
A second problem is that prolonged heating, contamination, and
thermal cycling increases brittleness and fragility and shortens
thermocouple life. Most metals, including those used in
thermocouples, become brittle with high-temperature exposure; as a
result, they can fail due to mechanical stresses induced by
vibrations, expansion, and contraction. Heat from welding to form
standard thermocouple junctions also can lead to mechanical failure
from embrittlement. Compatibility of component metals at high
temperature and improved joining methods are essential to improved
thermocouple durability.
It is an object of the invention to provide a high-temperature
thermocouple capable of operating in hostile environments for a
long period of time without significant signal degradation.
It is another object of the invention to provide a method of
fabricating a high-temperature thermocouple capable of operating in
hostile environments for a long period of time without significant
signal degradation.
It is still a further object of the invention to provide a
thermocouple capable of operating in a temperature range of
1100.degree. C. to 1700.degree. C. in a radiation environment.
Additional object, advantages and novel features of the invention
will become apparent to those skilled in the art upon examination
of the following and practice of the invention.
SUMMARY
Embodiments of the present invention relate to a design and method
for fabricating a thermocouple capable of operating for long
periods of time in a high-temperature, hostile environment without
significant signal degradation. Embodiments of the invention
include providing two dissimilar thermoelement wires having high
melting temperatures, high ductility and low neutron
cross-sections. The thermoelement wires are joined at one end by
swaging a metallic tube over the ends of the thermoelement wires to
form a thermocouple junction. In one embodiment of the invention
the thermoelement wires and junction with the insulation are heated
to remove moisture and impurities. The swaged thermocouple junction
helps avoid failure due to embrittlement associated with
conventional junction forming techniques. This junction is also
resistant to breakage due to metal fatigue and brittleness cause by
operating at high temperatures for long periods of time.
Embodiments of the invention utilize insulating materials that are
not reactive with the thermoelement wires or metallic sheath. In
one embodiment of the invention a less porous insulating material
is used to provide an interface with the insulating material
surrounding the thermocouple junction. A metallic sheath is loaded
with the insulating materials, thermoelement wires and thermocouple
junction, and then the metallic sheath is swaged to provide
intimate contact between the insulating materials and the
thermoelement wires and thermocouple junction. The interface
between the insulating materials is determined and the less porous
insulating material is physically removed by cutting the
thermocouple sheath at a location between the junction and the
interface. The sheath is then capped to provide a leak-tight
thermocouple. The thermocouple is then heated to a temperature for
a sufficient time to stabilize grain structure.
In one embodiment of the invention, the thermoelement wires are a
doped molybdenum and niobium/zirconium alloy. For example the
dopants of the molybdenum thermoelements are tungsten, potassium
and silicon in amounts typically ranging from 100 to 300 parts per
million. The niobium/zirconium alloy is 99% by weight niobium and
1% by weight zirconium. The metallic sheath is also comprised of
the same niobium/zirconium alloy.
BRIEF DESCRIPTION OF THE DRAWINGS
Illustrative embodiments of the invention are shown in the drawings
in which:
FIG. 1 is a photograph showing a thermocouple junction comprising a
metallic tube swaged onto thermoelement wires;
FIG. 2 is a photograph showing an electrical insulating material
having two interior passageways;
FIG. 3 is a photograph showing a second electrical insulating
material having a hollowed out void to receive the swaged
junction;
FIG. 4 is a sectional drawing illustrating the thermoelement wires,
thermocouple junction and insulating materials;
FIG. 5 is a sectional drawing illustrating the thermoelement wires,
thermocouple junction and insulating materials within a metallic
sheath; and
FIG. 6 is a graph showing the measured temperature over a period of
time of the present invention and conventional Type K and Type N
thermocouples.
DETAILED DESCRIPTION
Referring now to the drawings in which like numerals represent like
elements throughout the several views, embodiments of the present
invention will be described.
The present invention employs a thermoelement wire combination of
doped molybdenum and niobium/1% zirconium alloy with a sheath of
the same niobium/1% zirconium. The molybdenum is doped with the
elements tungsten, potassium, and silicon (typically dopants are
present in the range of 100-300 ppm). Molybdenum is an excellent
refractory metal but recrystallizes upon heating above 1200.degree.
C. The doped molybdenum selected for the present invention remains
ductile after heating for 12 hours at 1800.degree. C. Niobium has
excellent ductility, a high melting temperature, and low neutron
absorption. Alloying the niobium with zirconium increases its
recrystallization temperature. Importantly, molybdenum and niobium
are both less expensive than the metals used for conventional
high-temperature tungsten/rhenium or platinum/rhodium
thermocouples. Hence, in an embodiment of the invention, these
thermoelement materials are selected based on factors such as cost,
melting temperature, ductility, and low neutron cross-section.
During fabrication, care must be taken to avoid contaminating the
thermoelement wires, metallic junction tube, and sheath. This is
accomplished by cleaning these components with a solvent, such as
acetone, and handling the components with gloves.
Referring now to FIG. 1 a photograph of a metallic tube swaged onto
the thermoelement wires to form a thermocouple junction is shown.
The use of swaged metallic tubes, such as tantalum tubes, to form
the junction eliminates the requirement of a welded junction, which
have a tendency to become brittle and mechanically unstable at
higher temperatures.
Embodiments of the present invention may include hafnium dioxide
(HfO.sub.2) insulation paired with doped molybdenum and
niobium-zirconium alloy. When pairing hafnium dioxide (HfO.sub.2)
and niobium-zirconium alloy, tests indicate no discernible
interactions up to at least 1600.degree. C. (and material
properties suggest that higher temperatures are viable). The
chemical stability of the thermocouple/insulation pairing
contributes to signal stability of the present invention. The
insulating material of a thermocouple must have high electrical
resistivity and must not interact chemically with the thermoelement
wires or sheath materials. Even small changes it thermoelement
composition can result in decalibration. This is often due to
temperature-induced migration of impurities from the sheath and
insulation to the thermoelement wires. For standard thermocouples,
metal/insulation interactions are one cause of drift, but at
1600.degree. C., reaction kinetics make material interactions
difficult to avoid. Clearly, minimizing component material
interactions aids in minimizing thermocouple drift.
The electrical insulating materials are heated in a desiccator at a
high temperature for a sufficient time to outgas impurities and
moisture. The insulation may be heated to a temperature of at least
120.degree. C. for at least 24 hours. Heating at a higher
temperature will reduce the period of time needed for heating the
insulating material.
FIG. 2 shows the insulating material having two interior
passageways through which the ends of the thermoelement wires are
threaded. FIG. 3 is a photograph of a second insulating material in
which a void has been created by hollowing out an interior portion
of the insulating material. The void is formed to receive the
thermocouple junction.
A sectional drawing of the invention is shown in FIG. 4 that
illustrates a doped molybdenum thermoelement wire 12 and a
niobium/1% zirconium thermoelement wire 14 that have been threaded
through the first insulation material 16 passageways 18 and 20. The
first insulating material 16 is in contact with a second insulating
material 22. The first insulating material 16 has a void 23 formed
to receive the junction after the wires 12, 14 are threaded into
passageways 18 and 20. The second insulating material 22 has a void
24 formed therein for receiving a thermocouple junction 26.
FIG. 5 is a sectional drawing showing the first and second
insulating materials 16 and 22 respectively, containing the
thermoelement wires 12 and 14 within a metallic sheath 28. In
contact with the second insulating material 22 within the sheath 28
is a third insulating material 30. Sheath 28 is comprised of a
material that does not react with the insulating materials. Sheath
28 may be a niobium/zirconium alloy, such as a 99% niobium/1%
zirconium alloy. The third insulating material 30 is comprised of a
material that is less porous than the first and second insulating
materials 16 and 22. For example, the third insulating material 30
may be comprised of aluminum oxide (hereafter "alumina").
After insulators are threaded onto the thermoelement wires 12 and
14, the assembly is loaded into the Nb1% Zr sheath. A piece of
hafnium dioxide (hereafter "hafnia") insulating material 22 with a
void 24 is used to cover the junction. Then, a piece of alumina
insulation material 30 is placed above the hafnia insulation
material 22 to facilitate swaging. Experience indicates that the
less porous alumina prevents undesirable crushing of the junction
and thermoelement wires observed when only hafnia is placed within
the sheath. The sheath containing the junction, wire, and
insulation assembly is placed in an oven and baked for at least 8
hours at 120.degree. C.
The sheath 28 containing the thermoelement wires 12 and 14 and
insulating materials is then swaged to provide intimate contact
between the thermoelement wires 12 and 14, thermocouple junction 26
and the insulating materials 16 and 22. The swaging step results in
a repositioning of the interface between the second insulating
material 22 and third insulating material 30. The interface between
the second and third insulating materials 22 and 30 can be
ascertained by means of an x-ray imaging system or other
radiography techniques. In the present invention, the thermocouple
is cut orthogonally to its longitudinal axis at a location between
the junction and this interface between the insulating materials.
The end is then prepared for receiving an end cap (not shown) by
sanding the end of the sheath 28 flat. The end cap consists of a
niobium plug that is then laser welded to the cut end of the sheath
28.
The thermocouple is then checked for leak-tightness using a helium
leak detection system. As a final step in the fabrication of the
present thermocouple invention, a length that will encompass the
anticipated length that will see a temperature gradient during its
planned operating conditions is inserted into a tube furnace and
heat treated at an appropriate temperature above the anticipated
service temperature, for a sufficient duration to ensure assure
stable response. The thermocouple is typically heated at least
100.degree. C. above its planned service temperature for at least 3
hours.
Long duration tests in a high-temperature furnace at the Idaho
National Laboratory (INL) demonstrate that the present invention
remains stable with less than 2% drift after operating 4,000 hours
at 1200.degree. C. (see FIG. 6). Compare this with the leading
commercially available Type N and K thermocouples often used for
this temperature range that began to drift beyond 2% after only 200
hours at temperature. The present invention offers a twenty-fold
performance improvement in resistance to drift over this current
technology. Similarly, less than 2% drift was observed in present
invention tested at 1400.degree. C. for 4,000 hours, well beyond
the use range for Type N and K thermocouples.
For temperature ranges from 1100.degree. C. to 1700.degree. C., the
present invention competes well with more expensive
tungsten-rhenium or platinum-rhodium thermocouples that are prone
to failure after prolonged temperature exposure and repeated
thermal cycling. The enhanced performance of the present invention
is due to the ductility of its component metals and the unique
thermoelement joining method that results in a rugged sensor,
offering significantly longer, more stable service. The result is
fewer open-circuit failures (from fractures in the thermoelement
wires or at the junction) common to high-temperature thermocouples.
Finally, because the present invention is made from metals with
very low thermal neutron cross-sections, it can be used in nuclear
reactors without suffering decalibration due to neutron-induced
transmutation.
At temperatures above 1100.degree. C., the present invention is a
superior replacement for currently installed Type K and N
thermocouples with improved reliability, accuracy and longevity. At
higher temperatures (1100.degree. C. to 1700.degree. C.) the
invention offers even more advantages and can be more durable and
less prone to drift than competing Type B, C, D, R, or S
thermocouples. All of this can be achieved for a lower projected
per-unit price. The thermocouple of the present invention performs
well in this critical gap where other sensors often fail.
Furthermore, the invention is well suited for control of
long-duration, high-temperature processes with frequent thermal
cycling that often causes competing sensors to fail. For the
nuclear industry, the present invention thermocouple offers clear
advantages for affordable and reliable in-pile high-temperature
monitoring.
In compliance with the statute, the invention has been described in
language more or less specific as to structural and methodical
features. It is to be understood, however, that the invention is
not limited to the specific features shown and described, since the
means herein disclosed comprise preferred forms of putting the
invention into effect. The invention is, therefore, claimed in any
of its forms or modifications within the proper scope of the
appended claims appropriately interpreted in accordance with the
doctrine of equivalents.
* * * * *